GaiaFGK benchmark stars: new candidates at low metallicities

Abstract

Context. We have entered an era of large spectroscopic surveys in which we can measure, through automated pipelines, the atmospheric parameters and chemical abundances for large numbers of stars. Calibrating these survey pipelines using a set of "benchmark stars" in order to evaluate the accuracy and precision of the provided parameters and abundances is of utmost importance. The recent proposed set of Gaia FGK benchmark stars has up to five metal-poor stars but no recommended stars within −2.0 < [Fe/H] < −1.0 dex. However, this metallicity regime is critical to calibrate properly. Aims. In this paper, we aim to add candidate Gaia benchmark stars inside of this metal-poor gap. We began with a sample of 21 metalpoor stars which was reduced to 10 stars by requiring accurate photometry and parallaxes, and high-resolution archival spectra. Methods. The procedure used to determine the stellar parameters was similar to the previous works in this series for consistency. The difference was to homogeneously determine the angular diameter and effective temperature (T eff ) of all of our stars using the Infrared Flux Method utilizing multi-band photometry. The surface gravity (log g) was determined through fitting stellar evolutionary tracks. The [Fe/H] was determined using four different spectroscopic methods fixing the T eff and log g from the values determined independent of spectroscopy. Results. We discuss, star-by-star, the quality of each parameter including how it compares to literature, how it compares to a spectroscopic run where all parameters are free, and whether Fe i ionisation-excitation balance is achieved.Conclusions. From the 10 stars, we recommend a sample of five new metal-poor benchmark candidate stars which have consistent T eff , log g, and [Fe/H] determined through several means. These stars, which are within −1.3 < [Fe/H] < −1.0, can be used for calibration and validation purpose of stellar parameter and abundance pipelines and should be of highest priority for future interferometric studies.

TheGaia-ESO Survey

“…homogenous analysis relies on a rich set of calibrating objects (including GCs), selected as described by P16. In particular, the external calibration of FGK stars in iDR4 relies mostly on the Gaia benchmark stars Blanco-Cuaresma et al 2014;Heiter et al 2015a;Hawkins et al 2016). When comparing the iDR4 abundances obtained from UVES and GIRAFFE, small (i.e., comparable to the internal spreads) offsets in the abundance ratios were found (∼0.10-0.15 dex, depeding on the GC), as shown by P16.…”

TheGaiamission

“…For the RVS, a compilation of radial-velocity standards was undertaken (Crifo et al 2010;Soubiran et al 2013), while for the solar system object processing, data on solar analogues had to be compiled to calibrate the interpretation of the Gaia photometry. A set of ∼30 Gaia FGK benchmark stars have been extensively studied to provide a reference for the astrophysical parameter inference system Blanco-Cuaresma et al 2014;Heiter et al 2015;Jofré et al 2015;Hawkins et al 2016). One of the largest auxiliary observation programmes is the GBOT effort already mentioned in Sect.…”

A high-precision abundance analysis of the nuclear benchmark star HD 20

“…The corresponding analytical relations where trained and fit on the same training sample as the temperature method. The metallicity labels were either extracted from the detailed studies of the Gaia benchmark stars (Jofré et al 2014;Hawkins et al 2016), or stem from a homogeneous analysis of Fe ii lines in LTE (see Hanke et al 2018, for details). For HD 20, we found [Fe/H] = −1.62 ± 0.06 dex from the median and rms scatter of all 31 FRs, respectively.…”